1. Field of the Invention
The present invention relates to a video signal output circuit which amplifies and outputs video signals, and, in particular, relates to a technology which is effective when applied to a single power video driver which does not require an output coupling capacitor.
2. Description of the Related Art
An electronic apparatus such as a digital still camera or a DVD player is provided with a terminal to output video signals to an external display apparatus such as a liquid crystal display panel. As a device (IC) to output video signals to the terminal, a video driver is known. In recent years, in order to minimize an apparatus, a single power video driver which does not require an output coupling capacitor is practically used. According to the JEITA (Japan Electronics and Information Technology industry Association) standard, with respect to video signals outputted from a video apparatus, it is stipulated that a DC (Direct Current) voltage for when there is no signal is ±100 mV (75Ω, terminated). Hence, in a single power video driver, it is necessary to keep the DC voltage of an output terminal for when there is no signal within a range of 0 mV to 200 mV.
As a video driver which meets the condition described above, the present inventor et al. have developed, as shown in FIG. 8, a video driver including: a non-inverting amplifier AMP which amplifies video signals inputted from an input terminal IN within a predetermined range of amplitude; a clamp circuit CLP which makes the sync tip levels of horizontal synchronizing signals included in the video signals uniform; a resistance voltage dividing circuit DIV which generates a reference voltage Vref determining an operating point of the amplifier AMP, and also generates a bias voltage Vbias for the clamp circuit CLP; a buffer BUF for impedance conversion; and a low-pass filter (not shown). There is conventionally known a method of, in a video signal output circuit, fixing video signals at a predetermined electric potential by using a clamp circuit. Examples of the method are disclosed by, for example, Japanese Patent Application Laid-Open Publication No. sho 62-186674 and Japanese Patent Application Laid-Open Publication No. hei 7-183810.
As shown in FIG. 8, the resistance voltage dividing circuit DIV includes a resistor R1 and a resistor R2. The video driver shown in FIG. 8 can keep the DV voltage for when there is no signal within a range of ±100 mV centering 100 mV by setting a resistance value of the resistor R1 and a resistance value of the resistor R2 to R1−Roff and R2+Roff, respectively, so as to shift the bias voltage Vbias for a desired offset voltage (in this case, 100 mV which is the center of the range of 0 mV to 200 mV). The bias voltage Vbias is obtained by the resistance ratio of the resistor R1 to the resistor R2. The resistance value of the resistor R1 is set to R1−Roff in order to shift only the bias voltage Vbias, and not to shift the reference voltage Vref.
In the video driver shown in FIG. 8, the gain of the amplifier AMP is determined by the resistance ratio of a resistor Rs to a resistor Rf. However, by setting the resistance values of the resistors R1, R2, Rs, and Rf so as to be R1:R2=Rs:Rf, even when the resistance values of the resistors Rs and Rf shift from their respective desired resistance values because of manufacturing variations, the resistance ratio of the resistor Rs to the resistor Rf scarcely shifts. Furthermore, by setting the resistance values of the resistors R1, R2, Rs, and Rf so as to be R1:R2=Rs:Rf, when the resistance ratio of the resistor R1 to the resistor R2 shifts, the resistance ratio of the resistor Rs to the resistor Rf also shifts. Accordingly, the output DC level can be prevented from shifting, which occurs because of the manufacturing variations.